Thats correct. You have a light out most likely. If it happens sometimes then goes away, it may be a bad connection somewhere. You have to wait until its blinking fast again, then take a walk around the car for lights not working. When you find the one causing the rapid blinking, play around with its wiring. In my case its a rear light, next to the right break light.
It always decides to screw up when I'm trying to get my inspection sticker too...